Multi-carrier or Orthogonal frequency division multiplexing (OFDM) systems have gained considerable attention in the last few years. They are increasingly used in various wireless applications and communication systems. For instance, IEEE 802.11a standard and IEEE802.16 d/e standard provide an OFDM mode of transmission. The 802.16 d/e standard also provides multiple access based on basic OFDM ideas using a mode called OFDMA (Orthogonal Frequency Division Multiple Access).
In OFDMA systems, data is transmitted in a frame comprising a plurality of symbols. A symbol is a collection of narrowband sub-carriers transmitted in parallel at different frequencies from a same source. Each individual sub-carrier transmits information by modulating the phase and possibly the amplitude of the sub-carrier
Further, the frame in the OFDMA system can have multiple zones. For instance, OFDMA mode in IEEE 802.16e has a unicast zone and a Multicast and Broadcast Service (MBS) zone. Each zone comprises a group of symbols in the frame. Moreover, the frame comprises a preamble symbol for synchronizing with a receiver. The frame, also, comprises symbols in the unicast zone for communicating mapping information on a user's data bursts, known as downlink MAP (DL-MAP), to a receiver such as a mobile station. DL-MAP conveys information such as the presence of the MBS zone and the symbol index at which the MBS zone starts.
Generally, the plurality of symbols of the frame are received sequentially at the receiver. However, very often, the receiver may be interested in the data on a particular zone. For instance, the receiver may wish to decode data on the MBS zone, but not in the unicast zone. In some of the existing systems, the receiver may go into sleep or idle mode when it does not intend to transmit and receive any data for a particular time interval. During this time interval, almost all blocks of the receiver are shut down to reduce power consumption and hence the signals from the base stations are not being processed. However when it wakes up and intends to decode the MBS data, in existing implementations, it must first, necessarily, decode data in the unicast zone and then continue to decode the unicast zone over all frames until a frame with the MBS zone is encountered. In order to process the data in the unicast zone, the receiver performs many signal processing tasks such as channel estimation, frequency and timing synchronization, Quadrature Amplitude Modulation (QAM) demodulation, deinterleaving, channel decoding etc. These signal processing tasks consume power at the receiver.